Lurveleven,
Unfortunately the 19 mm will not work in a head designed for the 20. In the photo below is a comparison between the A-19 and A-2 heads along with the reflectors. The PD is essentially a A-2 in the optic end.
I built one PD proto with the 19 mm reflector and Cree 7090 UV LED. It is a sweet package and the head is about 7 mm longer than the production PD head.